Entangled particles are the bread-and-butter of quantum information schemes such as quantum cryptography, quantum computing, and quantum teleportation. But they are notoriously difficult to create in bulk. This issue may soon be resolved with word that researchers at the University of Oxford (Oxford, UK) have achieved laser-like amplification of entangled particles.1 Governed by quantum physics, entangled particles have much stronger correlations, or interrelationships, than anything allowed in classical physics. For example, measuring one entangled particle instantly influences its partner's state, even if the two particles are separated by great distances.
To create entangled photons, researchers can send laser light through a barium borate crystal. Upon passing through the crystal, a photon sometimes splits into two entangled photons (each with half the energy of the initial photon). However, this only occurs for one in every 10-billion incoming photons. To increase the yield, the Oxford researchers added a step. They put mirrors beyond the crystal so that the laser pulse and entangled pair could reflect, and have the chance to interact.
Since the entangled pair and reflected laser pulse behave as waves, quantum mechanics says that they could interfere constructively to generate fourfold more two-photon pairs or interfere destructively to create zero pairs. Following these steps, the researchers increased production of two-photon entangled pairs and also of rarer states such as four-photon entangled quartets. This achievement could represent a step towards an entangled-photon laser, which would repeatedly amplify entangled particles to create greater yields than previously possible, and also towards the creation of new and more complex kinds of entangled states.
1. Lamas-Linares et al., Nature, August 30, 2001